Thermal transfer printing has displaced impact printing in many applications due to advantages such as the relatively low noise levels which are attained during the printing operation. Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes and magnetic alpha-numeric characters. The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and color of the printed image. Representative documentation in the area of thermal transfer formulations and thermal transfer media used in thermal transfer printing includes the following patents.
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, discloses a thermal transfer medium having a coating composition of cellulosic polymer, thermoplastic resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,463,034, issued to Y. Tokunaga et al. on Jul. 31, 1984, discloses a heat-sensitive magnetic transfer element having a hot melt or a solvent coating.
U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, discloses a thermal transfer formulation that includes an adhesive-plasticizer or sucrose benzoate transfer agent and a coloring material or pigment.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,698,268, issued to S. Ueyama on Oct. 6, 1987, discloses a heat resistant substrate and a heat-sensitive transferring ink layer. An overcoat layer may be formed on the ink layer.
U.S. Pat. No. 4,707,395, issued to S. Ueyama et al., on Nov. 17, 1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer.
U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent.
U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8, 1990, discloses a thermal transfer ribbon which comprises two layers, a thermal sensitive layer and a protective layer, both of which are water based.
U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29, 1991, discloses a thermal transfer ribbon having a thermal sensitive coating and a protective coating. The protective coating is a wax-copolymer mixture which reduces ribbon offset.
U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer.
And, U.S. Pat. No. 5,240,781, issued to Obatta et al., discloses an ink ribbon for thermal transfer printers having a thermal transfer layer comprising a wax-like substance as a main component and a thermoplastic binder adhesive layer having a film forming property.
As demonstrated by the above patents, the selection of thermal transfer ribbons and formulations is diverse as are their end uses.
Thermal transfer printing has been widely used in printing characters for magnetic ink character recognition (MICR). Representative documentation in this area includes U.S. Pat. No. 5,041,331, issued to Glavin et al. on Aug. 20, 1991. U.S. Pat. No. 5,041,331 describes a thermal transfer ribbon with a functional layer with from 10 to 30 parts by weight magnetic metal oxide so that the resulting print will provide the desired level of signal transmission for machine scanning. While thermal transfer formulations and ribbons for MICR printing are known, inorganic metal oxide magnets are used to provide the necessary signal transmission for machine scanning. These inorganic metal oxides place limitations on the ribbon formulations and printed material produced. For example, the print obtained will always be opaque since the inorganic metal oxides are opaque. It is desirable to provide thermal transfer formulations, thermal transfer ribbons and MICR recognizable images which are not dependent on the use of inorganic metal oxides as magnets.
It is also desirable to provide coating formulations, thermal transfer media and MICR recognizable images which utilize unique magnetic pigments.
Organic molecule-based magnets are unique and have been disclosed in "Designer Magnets", Chemical & Engineering News, pp. 30-41, Oct. 2, 1995. Saturation magnetization values for organic molecule-based magnets have been reported as high as 21/2 times that of iron on a mole or metal-atom basis; however, the organic molecule-based magnets have a much lower density and higher molecular mass than iron. Therefore, the saturation magnetization of these materials on a mass or volume basis is far less than conventional magnets. Despite the lower saturation magnetization values, these compounds have received attention because their magnetic properties can be modulated by flexible organic syntheses and, because they are largely non-metallic, some have properties such as transparency and insulating behavior.
Examples include:
a) FeCp.sub.2 !.sup..multidot.+ TCNE!.sup..multidot., where PA1 Cp=pentamethylcyclopentadienide (C.sub.5 (CH.sub.3).sub.5) PA1 TCNE=tetracyanoethylene (NC).sub.2 C =C(CN).sub.2 PA1 b) 4-nitrophenylnitronyl nitroxide PA1 c) 1,3,5,7-tetramethyl-2,6-diazaadamantane-N,N'-dioxyl PA1 d) FeCp.sub.2 !.sup..multidot.4 TCNE!.sup..multidot. PA1 e) FeCp.sub.2 !.sup..multidot.4 TCNQ!.sup..multidot. PA1 f) MnCp.sub.2 !.sup..multidot.4 TCNE!.sup..multidot. PA1 g) FeCp.sub.2 !.sup..multidot.+ TCNE!.sup..multidot. PA1 h) Mn.sup.III TPP!.sup.+ TCNE!.sup..multidot. PA1 i) (CrCp.sub.2).sup..multidot.4 TCNE!.sup..multidot. PA1 TPP=meso-tetraphenyl porphyrin PA1 j) V(TCNE).sub.2 1/2CH.sub.2 Cl.sub.2 (obtained by reacting V(C.sub.6 H.sub.6).sub.2 or V(CO).sub.6 and TCNE.